High performance flow battery

ABSTRACT

High performance flow batteries, based on alkaline zinc/ferro-ferricyanide rechargeable (“ZnFe”) and similar flow batteries, may include one or more of the following improvements. First, the battery design has a cell stack comprising a low resistance positive electrode in at least one positive half cell and a low resistance negative electrode in at least one negative half cell, where the positive electrode and negative electrode resistances are selected for uniform high current density across a region of the cell stack. Second, a flow of electrolyte, such as zinc species in the ZnFe battery, with a high level of mixing through at least one negative half cell in a Zn deposition region proximate a deposition surface where the electrolyte close to the deposition surface has sufficiently high zinc concentration for deposition rates on the deposition surface that sustain the uniform high current density.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 14/470,779, filed on Aug. 27, 2014, which is acontinuation application of U.S. patent application Ser. No. 13/076,337,filed on Mar. 30, 2011 and now published as US 2011/0244277, whichclaims benefit of U.S. Provisional Patent Application Ser. No.61/319,248, filed on Mar. 30, 2010 and U.S. Provisional PatentApplication No. 61/322,780, filed on Apr. 9, 2010, all of which areincorporated herein by reference in their entirety herein.

FIELD OF THE INVENTION

This invention relates to high performance electrochemical cells andbatteries, and more particularly to flow batteries.

BACKGROUND OF THE INVENTION

The “greening” of the energy economy, increasing demand and use ofrenewable energy sources such as wind and solar, and the expectedproliferation for example of plug-in hybrid vehicles and all electricvehicles, increasingly strain the electricity distribution grid. Highcapacity electrical energy storage technologies such as pumpedhydroelectric can play an important role in grid load balancing, timeshifting renewable energy sources from time of generation to peak timeof use, however, geography and cost limit their use, particularly on alocal level.

Existing high capacity battery technologies, for example flow batteries,are too expensive for widespread adoption because the effective cost ofthe resulting energy and/or power delivered is well above market prices.There exists therefore a substantially unmet need for a low-cost, highcapacity, efficient and high performance battery technology.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide high performance flowbattery apparatus and methods for enhancing, charging, operating andusing flow batteries. High current density charging rates anddischarging rates in the range of approximately 70 to 400 mA/cm², andmore particularly in the range of 100 to 250 mA/cm², are provided byvarious embodiments of the present invention.

Embodiments of the high performance, alkaline zinc/ferro-ferricyaniderechargeable (“ZnFe”) flow batteries of the present invention are basedon a number of improvements over the prior art. These embodiments arealso applicable to other flow batteries that incorporate the plating ofa metal to store energy (such as: ZnHBr; ZnBr; CeZn; and ZnCl).

First, the battery design has a cell stack comprising a low resistancepositive electrode in at least one positive half cell and a lowresistance negative electrode in at least one negative half cell, wherethe positive electrode and negative electrode resistances are selectedfor uniform high current density across a region of the cell stack—thatis with a resistance across the electrodes sufficiently low to ensuresmall voltage variations across the electrode and hence uniform currentflow out of the electrode and across the cell stack.

Second, a flow of electrolyte (for example, zinc species in the ZnFebattery) with a high level of mixing (also referred to herein as a “highrate of mixing” and “high mixing”) through at least one negative halfcell in a Zn deposition region proximate a deposition surface where theelectrolyte close to the deposition surface has sufficiently high zincconcentration for deposition rates on the deposition surface thatsustain the uniform high current density. The electrolyte flow andmixing of the flow in the negative half cell are engineered to provide amass transfer coefficient sufficient to support the high current densityand to provide substantially uniform deposition of, for example zinc,over the deposition surface of a cell. Furthermore, some embodimentshave been flow engineered to provide zinc deposition at less than alimiting current, where the deposited zinc has a dense, adherent,non-dendritic morphology.

Third, the zinc electrolyte has a high concentration and in someembodiments has a concentration greater than the equilibrium saturationconcentration—the zinc electrolyte is super-saturated with Zn ions.Different embodiments of the present invention combine one or more ofthese improvements.

Electrolyte flow with high mixing through the cell may be due to highfluid velocity in a parallel plate channel. However, the mixing in theflow may be induced by structures such as: conductive and non-conductivemeshes; screens; ribbons; foam structures; arrays of cones, cylinders,or pyramids; and other arrangements of wires or tubes used solely or incombination with a planar electrode surface. Use of such structures mayallow for high mixing of the electrolyte with laminar flow or withturbulent flow at high or low fluid velocity. Furthermore, structuresfor calming the turbulent flow may be included in the electrolyte fluidcircuit immediately after the cell.

According to embodiments of the present invention, methods for operatinga flow battery may include flowing electrolyte with high mixing in alaminar flow regime, or turbulent flow regime, through at least onenegative half cell in a Zn deposition region proximate a depositionsurface. Furthermore, some embodiments include depositing Zn with adense, adherent, non-dendritic morphology. The high mixing flow may beutilized during charging and/or discharging of battery cells.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a schematic diagram of a zinc redox flow battery;

FIG. 2 is schematic diagram of a zinc redox flow battery, according tosome embodiments of the present invention;

FIG. 3 is a schematic perspective view of a flow cell, according to someembodiments of the present invention;

FIG. 4 is a schematic perspective view of the cell of FIG. 3 containedwithin a frame, according to some embodiments of the present invention;

FIG. 5 is a schematic cross-sectional representation of a first exampleof cell configurations for a redox flow battery, according to someembodiments of the present invention;

FIG. 6 is a schematic cross-sectional representation of a second exampleof cell configurations for a redox flow battery, according to someembodiments of the present invention;

FIG. 7 is a schematic cross-sectional representation of a third exampleof cell configurations for a redox flow battery, according to someembodiments of the present invention;

FIG. 8 is an example of a mixing inducing woven wire mesh feature on thesurface of a flow battery electrode, according to some embodiments ofthe present invention;

FIG. 9 is an example of a mixing inducing non-woven wire mesh feature onthe surface of a flow battery electrode, according to some embodimentsof the present invention;

FIG. 10 is an example of a mixing inducing wire/tube feature on thesurface of a flow battery electrode, according to some embodiments ofthe present invention;

FIG. 11 is an example of a mixing inducing array of cylinders on thesurface of a flow battery electrode, according to some embodiments ofthe present invention;

FIG. 12 is an example of a mixing inducing array of cones on the surfaceof a flow battery electrode, according to some embodiments of thepresent invention;

FIG. 13 is an example of a mixing inducing array of pyramids on thesurface of a flow battery electrode, according to some embodiments ofthe present invention; and

FIG. 14 is a cross-sectional representation of a flow laminarizationfeature, according to some embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of some embodiments of the invention so as to enable thoseskilled in the art to practice the invention. Notably, the figures andexamples below are not meant to limit the scope of the present inventionto a single embodiment, but other embodiments are possible by way ofinterchange of some or all of the described or illustrated elements.Moreover, where certain elements of the present invention can bepartially or fully implemented using known components, only thoseportions of such known components that are necessary for anunderstanding of the present invention will be described, and detaileddescriptions of other portions of such known components will be omittedso as not to obscure the invention. In the present specification, anembodiment showing a singular component should not be consideredlimiting; rather, the invention is intended to encompass otherembodiments including a plurality of the same component, and vice-versa,unless explicitly stated otherwise herein. Moreover, applicants do notintend for any term in the specification or claims to be ascribed anuncommon or special meaning unless explicitly set forth as such.Further, the present invention encompasses present and future knownequivalents to the known components referred to herein by way ofillustration.

Embodiments of the present invention provide high performance flowbattery apparatus and methods for enhancing, charging, operating andusing flow batteries.

FIG. 1 shows an example of a prior art redox flow battery 100. See, forexample, Wu et al. Indian Journal of Technology, vol. 24, July 1986, pp372-380. The flow battery comprises positive and negative half cells 110and 120, respectively, separated by separator 130. Electrolyte for thehalf cells is stored in tanks 140 and 150 and is pumped through the halfcells, as shown by the arrows. The flow battery shown in FIG. 1 is aZn/Fe redox flow battery; the posilyte is an Fe complex and the negalyteis a zincate salt. However, prior art flow batteries do not operate athigh enough current densities and are not efficient enough to beeconomically viable for large scale energy storage. The presentinvention provides improvements to flow batteries that will allow highcurrent density operation with high efficiency at low cost. For example,some embodiments of the present invention will provide redox flowbatteries with charging current densities of 70, 80, 90, 100, 125, 150,200 mA/cm² and even higher.

The alkaline zinc/ferro-ferricyanide (“ZnFe”) rechargeable batterysystem of some embodiments of the present invention is intended forutility load leveling, load following, area regulation services,transmission & distribution deferral applications, wind and solarintegration applications amongst other megawatt energy storageapplications having an energy storage capacity from a few minutes, suchas 15 minutes up to and exceeding 24 hours duration. The ZnFe battery isa hybrid redox flow battery in which the active materials (zinc oxideand sodium ferrocyanide) are stored in reservoirs external to the celland brought to the site of electrochemical reaction as saturatedsolutions in a sodium hydroxide electrolyte.

During charge, energy is stored in the form of zinc metal deposited uponthe zinc electrode substrate and as ferricyanide formed by anodicoxidation of the ferrocyanide reactant. When the demands of the loadrequire, energy may be drawn from the cell by anodically dissolving thezinc to form zinc oxide with the simultaneous reduction of ferricyanideions to ferrocyanide. These processes are highly reversible andselective, enabling the cell to operate with the advantages of highcycling efficiency, high cell voltage, random cycling and switch timesof less than 5 ms from load to isolation or from isolation to full load.

Prior art flow batteries, especially Zn based, have problems withdendrite growth particularly as operating current density is increasedduring charging (deposition). For example, zinc dendrites may formduring the deposition (charging) process in a zinc-based battery due tovarious causes. Zinc dendrites can cause problems in zinc-basedbatteries including a reduction in performance, cell short circuits andreduced operating lifetime all of which increase effective operatingcosts.

Embodiments of the present invention will provide higher performance(and thereby a lower operating cost) for zinc and other flow batteriesby increasing the sustainable operating current density for charging anddischarging of cells with reduced or minimized growth of dendrites. Flowbattery embodiments of the invention, particularly for grid storageapplications, generally will have power outputs approximately in therange from 20 kW to 25 MW and greater and energy outputs approximatelyin the range of 5 kWh to 600 MWh or discharge durations from 5 to 15minutes to over 24 hours for a given power rating of the flow batteryalthough higher and lower power and energy outputs can be used.Generally, the charge and discharge times are defined by the marketapplication for a specific flow battery product. Typical discharge timesare 15 minutes, 1, 2, 4, 8, 12, 16 and 24 hours. The ratio of charge todischarge time is generally in the range from 2 to 1 or 1 to 1 or 1 to2, with approximately a 1 to 1 charge to discharge ratio beingdesirable.

Embodiments of high performance flow batteries, for example a ZnFe flowbattery, of the present invention are based on a number of improvementsover the prior art that will allow operation at high current densitiesand/or that lower battery overall operating costs.

First, the battery design has a cell comprising a low resistancepositive electrode in at least one positive half cell and a lowresistance negative electrode in at least one negative half cell, wherethe positive electrode and negative electrode resistances are selectedfor uniform high current density across a region of the cell stack—thatis with a resistance across the electrodes sufficiently low to ensuresmall voltage variations across the electrode and hence uniform currentflow out of the electrode and across at least a region of the cell (forexample, voltage variations typically less than 5 to 10 mV where theresistance across a cell results in less than 200 mV loss at anoperating current density of 100 mA/cm², corresponding to a variation incurrent density of less than 20%.) Cells are often assembled together inseries in a cell stack that includes multiple cells. The electricalconnection between cells in the cell stack can be in the form of abipolar electrode or other electrode designs including using wires toconnect cells together in series and or parallel to make a cell stack.Typically multiple cell stacks are combined to make a battery system.

Second, a flow rate of electrolyte (for example, zinc species in theZnFe battery) with a high rate of mixing is induced through at least onenegative half cell in a Zn deposition region proximate a depositionsurface where the electrolyte solution has sufficiently high zincconcentration for deposition rates on the deposition surface thatsustain the uniform high current density across a cell or acrosssubstantially all of the cells in a cell stack. The flow in the negativehalf cell is engineered to provide substantially uniform deposition ofzinc over the deposition surface. Furthermore, some embodiments are flowengineered to provide zinc deposition, where the zinc has a dense,adherent, non-dendritic morphology. The flow may be laminar with mixingelements or the mixing may be achieved through turbulent flow at highvelocity or turbulent flow at lower velocity with turbulence elementsadded to a flow channel of the cell.

Third, the zinc electrolyte has a high concentration and in someembodiments has a concentration greater than the equilibrium saturationconcentration, that is the zinc electrolyte is super-saturated with zincions. Different embodiments of the present invention combine one or moreof these improvements.

The flow battery operating current density is a function of theconcentration of active ion species. Some embodiments of the inventionprovide a super-saturated electrolyte to increase the concentration ofions particularly during charging. Zincate electrolyte can bemanufactured with super-saturated zinc (Zn) ions through a chemical orelectrochemical route. For example, zincate electrolyte can bemanufactured with approximately ˜1 to ˜1.9 Molar zinc ions, whichremains stable for in excess of one day. See Dirkse, Journal of theElectrochemical Society, Volume 128 (No. 7), July 1987, pp 1412-1415;Dirkse, Journal of the Electrochemical Society, Volume 134 (No. 1),January 1987, pp 11-13; and Debiemme-Chouvy & Vedel, Journal of theElectrochemical Society, Volume 138 (No. 9), September 1991, pp2538-2542. Note that it is permissible to have zincate particles in theelectrolyte provided that the particle size is small relative to thesize of the electrolyte channel, that is, the flow channel of the cell.Furthermore, the electrolyte chemistry for the ZnFe flow battery has theadded advantage of providing basic (high pH) electrolytes, which areless corrosive than many of the alternative electrolyte chemistries,which are more acidic. A basic chemistry is advantageous for the initialcost and longevity of components of the flow battery such as theplumbing and pumps used to feed the electrolyte flow to and from thecell stack of the flow battery.

High operating current density across the cell deposition surface andthrough the cell stack lowers the effective cost per unit power orenergy output of the battery and lowers overall operating costs.Embodiments of the invention will provide sustainable higher operatingcurrent density by ensuring that dendrite growth is avoided or minimizedparticularly during charging (deposition).

Dendrite growth will be avoided or minimized by ensuring generallyuniform operating current density across the deposition surface in thecell and by ensuring there is always an adequate, generally uniform andhigh concentration of ions in the electrolyte available at or close tothe cell deposition surface where the ion concentration is consistentwith the high operating current density and sufficient or greater thanthe concentration required to sustain the current density throughdeposition surface(s).

High current density operation with laminar flow of electrolyte throughthe cell flow channel without adequate mixing results in reduced ionconcentration in the diffusion boundary layer at or close to thedeposition surface, which results in non-uniform deposition and dendritegrowth. Operating the cell with an electrolyte flow regime that resultsin mixing (either with laminar flow or with turbulence) in theelectrolyte flow through the cell flow channel increases the masstransfer coefficient and decreases the diffusion boundary layerthickness at the deposition surface which in turn increases theavailability of ions for deposition. High availability of ions (forexample zinc ion concentration in zincate in a ZnFe battery) allowshigher current density operation without significantly depleting theelectrolyte concentration in the uniform region of the cell depositionsurface(s) and as a result with little or no dendrite growth.

The combination of both increased zincate ion concentrations in theelectrolyte and increased mixing of the electrolyte in the cell flowchannel near the deposition surface, both relative to prior art cells,will reduce or eliminate the formation of dendrites. This will allowsustainably increased high current density operation and will result ina smaller sized cell, smaller overall cell stack and smaller overallmodule which will decrease the cell, stack and module costs and overalloperating costs for a given power and/or current output. These resultingcapabilities will provide a more economic battery system and will lowerthe overall cost of energy and power output of a battery system.

Cell performance is enhanced by engineering the electrolyte flow andcell flow channel geometry to generate sufficient mixing or turbulenceto reduce the diffusion boundary layer thickness at the depositionsurface.

Tables 1 and 2 below shows illustrative values of high operating currentdensity and associated average mass transfer coefficient (k_(m))estimates for the flow in the cell flow channel according to embodimentsof the present invention. The mass transfer coefficient relates the rateof mass transfer to the electrode surface (mol/cm²·s) and the differencein concentration between the bulk of the solution and at the electrodesurface (mol/cm³). Mixing in the cell flow channel for increasedoperating current density can also be described in terms of the SherwoodNumber or mean Sherwood Number (Sh_(m)) defined as the dimensionlessmass transfer coefficient, also defined as the ratio of convectivetransport to diffusive transport of ions in the electrolyte. Note thatthe examples of Sherwood numbers in the tables below are calculatedbased on correlations for flow through 3D turbulent structures; however,other calculation methods may be used within the spirit and scope of thepresent invention. i_(L) is the limiting current density, that is thecurrent density at zero ion concentration in mA/cm² at the electrodesurface (or electrode solid interface). i_(app) is the favorable celloperating current density, defined for purposes of the examples in Table1 as approximately ˜⅔ times i_(L) in mA/cm² (although those of skill inthe art will recognize that other values or definitions may be usedwithin the spirit and scope of various embodiments of the invention). vis the average flow velocity in cm/s of the electrolyte flowing throughthe cell flow channel. C_(b) is the bulk concentration, i.e. the activeion concentration outside the diffusion boundary layer, mol/l. Tables 1and 2 below also provide illustrative examples of these parameters.While these parameters and terms are familiar to those skilled in theart, additional details can be found in text books such as for example“Advanced Transport Phenomenon: Fluid Mechanics and ConvectiveTransport” by L. Gary Leal Chapter 9, published by Cambridge UniversityPress, in 2007, and “Unit Operations of Chemical Engineering” by WarrenL. McCabe, Julian C. Smith and Peter Harriot, Chapter 21, published byMcGraw Hill Inc (V^(th) edition, 1993).

TABLE 1 Operational Range Examples with C_(b) = 0.25 (mol/L) I_(app)(mA/cm²) 70 100 150 200 250 400 C_(b) (mol/L) 0.25 0.25 0.25 0.25 0.250.25 i_(L) (mA/cm²) 105 150 225 300 375 600 k_(m) (cm/s) 2.3 × 10⁻³ 3.1× 10⁻³ 4.6 × 10⁻³ 6.2 × 10⁻³ 7.8 × 10⁻³ 12.4 × 10⁻³ Sh_(m) 64 86 129 172215 342

TABLE 2 Operational Range Examples with C_(b) = 1 (mol/L) I_(app)(mA/cm²) 70 100 150 200 250 400 C_(b) (mol/L) 1.0 1.0 1.0 1.0 1.0 1.0i_(L) (mA/cm²) 105 150 225 300 375 600 k_(m) (cm/s) 5.3 × 10⁻⁴ 7.7 ×10⁻⁴ 1.2 × 10⁻³ 1.5 × 10⁻³ 1.9 × 10⁻³ 3 × 10⁻³ Sh_(m) 16 21 32 43 54 86

For cell operation in the approximate range of 70 to 400 mA/cm² thedesirable mass transfer coefficient is between approximately 5×10⁻⁴ and1.24×10⁻² cm/s. For cell operation in the approximate range of 70 to 400mA/cm² the desirable mean Sherwood number is between approximately 15and 350.

Calculated zinc deposition thickness on the cell deposition surface(s)for 8 hours of charging operation for the following high currentdensities are shown in Table 3.

TABLE 3 Approximate Zinc Deposition Thickness for 8 Hours of Chargingcurrent density (mA/cm²) 100 200 400 deposit thickness (cm) 0.17 0.340.68 deposit capacity (mAh/cm²) 800 1,600 3,200 mass of deposit (g/cm²)0.976 1.951 3.902

Note that these thickness figures scale linearly with current densityand time. For example, for a current density of 100 mA/cm², a growthrate of approximately 0.21 mm/hour is calculated; for a current densityof 200 mA/cm², a growth rate of approximately 0.43 mm/hour iscalculated; and for a current density of 400 mA/cm² a growth rate ofapproximately 0.85 mm/hour is calculated.

Although the examples provided herein are of ZnFe redox flow batteries,other redox flow batteries may be fabricated using the teaching andprinciples of the present invention. For example, the followingbatteries may be fabricated: ZnHBr; ZnBr; CeZn; and ZnCl.

FIG. 2 shows a schematic representation of a flow battery 200, accordingto some embodiments of the present invention. FIG. 2 is an example of aZnFe flow battery. The flow battery 200 has a positive electrode 212, anegative electrode 222 on the surface of which there is a zinc platingzone 224, and a membrane 210 separating a positive channel 211 and anegative channel 221. The flow of electrolyte through the separatechannels in the cell and through the rest of the fluid circuits isindicated by arrows 213 and 223, for the posilyte and negalyte circuits,respectively. Each fluid circuit includes a cell channel (211 and 221),an optional flow calming feature 262 (such as shown in FIG. 14), anelectrolyte reservoir (240 which in this example contains theposilyte-sodium ferrocyanide/ferricyanide solution and 250, which inthis case contains the negalyte sodium zincate solution), a pump 263 andflow sensor(s) 264. The pumps 263, sensors 264 and pump controller 265are configured to control the flow of electrolyte through the cell,including control of the flow regime in the cell—the flow regime may belaminar, mixing, and/or turbulent flow as described throughout thisapplication.

The amount of electrolyte pumped through each side of theelectrochemical cell is determined by calculation of the cell channelvolume, the rate of electrolyte flow and the amount of zincate depletiondesired at the cell exit chamber with consideration for the energystorage duration and piping volumes and pipe run lengths defining theelectrolyte storage tank sizes. For a given amount of electrolyte percell stack, a pump size is selected using materials of constructionhaving durability and long life under conditions of strong base (e.g. 2to 5 N NaOH) or strong acid depending upon the nature of the electrolyteused in the electrochemical cell. Typically, two pumps are selected foreach cell stack, one for each electrolyte. Generally acceptablematerials of construction include polypropylene, polyethylene,fluorinated polymers, polyetherketones, polysulfones, polyphenylenesulfide and the like. Various sensors are selected to measure the fluidvelocity, direction of fluid flow, temperature, pressure and othermetrics at various locations in the storage tanks, piping, pumps,entrance and exit points of the cell channel. The various data signalsfrom each sensor are transferred by signal wire or by wireless transferto a data control system. The data control system records the data flow,and uses algorithms, set points and control inputs to send data signalsto fans (for cooling, if required), valves and motors to control (e.g.increase, decrease or hold) motor speeds and valve positions which inturn increase, decrease, hold constant or change fluid directions oncommand. The data control system may under certain conditions send alarmsignals and other performance data to remotely located control rooms.The piping to and from the cell channel is designed and sized tominimize shunt current loses and the materials of construction areselected for durability under conditions of strong base or strong aciddepending upon the nature of the electrolyte. The electrochemical systemis generally sited inside a fluid containment system comprisingappropriate sensors and alarms to indicate any electrolyte leaks.

FIG. 3 shows a schematic perspective view of a cell 300. The cell 300may be roughly 0.5 cm thick, with larger dimensions of roughly 30 cm×30cm up to 132 cm×67 cm, for example. A cross-sectional view of thesection X-X is shown in FIG. 7. FIG. 3 shows a cell 300 with bipolarstructural elements on either side. The cell has positive 211 andnegative 221 channels separated by a membrane 230. A negalyte is pumpedthrough the negative channel and a posilyte is pumped through thepositive channel, as shown; posilyte fluid flow and negalyte fluid floware shown by arrows 213 and 223, respectively. Further details of thecell are provided above with reference to FIG. 7.

FIG. 4 shows the cell 300 of FIG. 3 in a frame 410. The cell 300 issurrounded by the frame 410 that serves to hold the membrane (separator)and bipolar structural elements in place, creating the flow channels,sealing the edges of the flow channels, providing a place to attach theelectrolyte flow and return pipes and, optionally, may containelectrolyte distribution manifolds and flow calming features. Flow ofposilyte and negalyte into and out of the frame for provision to thecell 300 is indicated by arrows 213 and 223, respectively.

More detailed examples of cells according to some embodiments of thepresent invention are provided in FIGS. 5-7. The cells may have largedimensions of 30 cm×30 cm, 90 cm×90 cm, 60 cm×90 cm, 45 cm×90 cm, or 132cm×67 cm, for example. Examples of the cross-sectional dimensions of thecomponents of the cells are provided in FIGS. 5-7. However, the presentinvention is not limited to these cell dimensions and may be used withcells of smaller or larger dimensions. The cells are shown incross-section, and the section is perpendicular to the larger surface ofthe cell. (For example, see section X-X in FIG. 3.) The power density isa function of the cell chemistry and the cell current density. For ZnFeat 200 mA/cm², for example, the discharge power density is about 0.3W/cm². For the cell dimensions listed above, the resulting power percell will be approximately 274 W, 2.43 kW, 1.60 kW, 1.22 kW and 2.45 kW,respectively.

FIG. 5 shows a first example of a schematic cross-section of a bipolarZnFe redox flow battery cell of the present invention. A single batterycell is shown comprising a negative half cell 220, a positive half cell210 and a bipolar structural element 270. The bipolar structural element270 separates the positive half cell from the negative half cell ofadjacent cells. (See FIG. 6.) The bipolar structural element 270 in thisexample is a 50% graphite fiber/PPS interconnector on which there is acadmium metal strike 271. (PPS is polyphenylene sulfide. Other polymermaterials can be used in place of PPS in the construction of bipolarstructural elements, such as polyetherketones, polysulfones,polyethylenes, polypropylenes and the like in combination withconductive fillers such as graphite fiber or flakes, certain carbonpowders and carbon blacks, carbon nanotubes, conductive metal powders,and the like.) The positive half cell 210 comprises a porous Ni meshredox electrode, which completely fills the positive channel 211—theposilyte flows through the porous Ni mesh redox electrode. The negativehalf cell 220 includes a Zn plating zone 224 of variable thickness onthe Cd metal strike 271 and a negalyte flow channel 221. The positivehalf cell and the negative half cell of an individual cell are separatedfrom each other by a membrane 230, made of material such as Nafion-114or another separator material. The membrane 230 keeps the zincate andiron electrolytes separated, but Na ions and water are able to movethrough the membrane. The membrane material can be a separator materialwith or without grafted ionic chemical species.

In order to operate the bipolar cell of FIG. 5 at a high currentdensity, for example a charging current density of 200 mA/cm², a highmass transfer rate is generated in the negative flow zone. This may beachieved by increasing the mixing rate and/or by increasing theelectrolyte fluid flow rate, above that of prior art cells. This may bedone by adding mixing elements to the cell channel, or by increasing thevelocity without reaching the turbulent flow regime, or by increasingthe velocity until turbulent flow is achieved, or by introducingturbulence generating elements as discussed below. Note that zincdeposition current density is a function of fluid (electrolyte) velocityand Reynolds number. See R. D. Naybour, “The Effect of Electrolyte Flowon the Morphology of Zinc Electrodeposited from Aqueous AlkalineSolution Containing Zincate Ions” J. Eletrochem. Soc. pages 520-525,April 1969. Note that the deposition operating current density is afunction also of the concentration of the active species.

FIG. 6 shows a second example of a schematic cross-section of a bipolarZnFe redox flow battery cell. The positive electrode 612 comprises aporous Ni mesh redox electrode attached to a bipolar Ni/Cu electrode272—the Ni mesh being attached to the Ni face of the bipolar electrode.The posilyte flow zone is occupied by the porous Ni mesh. The negativeelectrode 622 may comprise a Cd, Sn or Pb coated high surface area Cu orNi mesh, which is 60% to 98% porous, for example. The coated Cu or Nimesh is attached to the Cu face of the bipolar electrode 272. The coatedCu or Ni mesh occupies the negalyte flow zone and the mesh generatesmixing in the negalyte flow, without requiring high fluid velocity. Thecell is set up so that the coated Cu or Ni mesh may be plated with Zn upto approximately 20% to 70% of volume. FIG. 6 also illustrates how thebipolar electrode 272 (or the bipolar structural element 270 of FIG. 5)separates the cell from adjacent cells and facilitates the efficient andcost-effective construction of a cell stack. Adjacent cells are shown inFIG. 6.

FIG. 7 shows a third example of a schematic cross-section of a bipolarZnFe redox flow battery cell. (FIG. 7 is section X-X in FIG. 3.) Thepositive channel 211 comprises a porous Ni mesh redox electrode attachedto a bipolar Ni/Cu electrode 272—the Ni mesh being attached to the Niface of the bipolar electrode. The posilyte flow zone is occupied by theporous Ni mesh. The negative channel 221 includes a zinc metal platingzone 224 and features 280 configured to induce efficient mixing orturbulence. Examples of features 280 are shown in FIGS. 8-13 and aredescribed below. (Note that features 280 may be positioned in the flowchannel above the deposition surface as shown in FIG. 7, or in otherembodiments may be positioned directly on the deposition surface, asshown in FIGS. 11-13—FIGS. 8-10 show structures that may be eitherpositioned on the deposition surface or above it.) These features aredesigned to generate a high rate of mixing of the flow while notnecessarily requiring high velocity (and therefore high pumping powerdissipation). Zn metal is plated on the Cu face of the bipolar electrode272. The Cu face may alternatively also be coated with Cd, Sn or Pb.Note that the cylinders, cones and pyramids shown in FIGS. 11-13 will bemade of non-conducting material and are shown to have sharp edges andpoints. However, if it is desired to make the cylinders, cones andpyramids of conductive material then they should have blunt edges andends rather than sharp edges and points. (Note that in order to improvethe uniformity of Zn plating, the features used to induce mixing and/orturbulence should not have sharp points or edges if they areconductive—sharp points and edges are electric field concentrators andlead to undesirable non-uniform plating and even dendrite formation.)Furthermore, the features should not occupy too large a volume such thatflow through the anode channel is unduly impeded—see below for furtherdetails.

Note that the conditions under which turbulent flow is induced can beconveniently defined for a particular channel geometry by using, forexample Reynolds numbers. Those skilled in the art are familiar with thecalculation of Reynolds numbers, including for channels that containfeatures such as those shown in FIGS. 8-13. Roughly, for the cellillustrated in FIG. 7 with a substantially rectangular channel, aReynolds numbers of at least approximately greater than 1,300 orpreferably 2,000 may be used to ensure turbulent flow where thecharacteristic length is defined through hydraulic diameter. Thehydraulic diameter for a narrow flow channel (L>>W) is twice thethickness of the channel, i.e. 2W, where L and W are the length andwidth of the flow channel, as measured perpendicular to the direction offluid flow. (See FIG. 3.)

For the cell shown in FIG. 7 incorporating the features shown in FIGS.8-13, lower Reynolds number may suffice to ensure efficient mixing or ahigh rate of mixing, for example Reynolds numbers of at leastapproximately 8 or greater may suffice to ensure efficient mixing.However, the specific Reynolds number will vary with the cell and flowchannel design and mixing feature.

FIG. 8 shows an example of a structure suitable for inducing mixing orturbulence in the electrolyte flow over the surface of a flow batteryelectrode. A small section of woven wire mesh 820 is shown on part ofthe surface of the electrode 810. The direction of electrolyte flow isindicated by the arrow; the flow being generally parallel to the surfaceof the electrode 810. The wire mesh 820 disrupts the fluid flow,inducing desirable mixing in either laminar, semi-turbulent or turbulentflow over the electrode surface. Note that a similar effect may also beachieved by having the mesh close to, but not necessarily on, thesurface of the electrode 810. A suitable wire diameter will be between20% and 50% of the channel thickness.

In some embodiments the wire mesh is conductive and acts as part of theelectrode surface thereby increasing the total electrode surface area(in addition to the mesh acting as a mixing element). In otherembodiments, the mesh is non-conductive and mixes the flow on thesurface of the planar electrode. A non-conductive mesh can also be usedto ensure a specified electrode-to-membrane spacing when it issandwiched between the electrode and membrane. In yet other embodiments,there can be several layers of mesh, some conductive and somenon-conductive. In one such example, a conductive mesh is adjacent tothe electrode and a non-conductive mesh is between the conductive meshand the membrane. The non-conductive mesh acts as a spacer to keep theplating surfaces away from the membrane, as well as, acting as a flowmixing structure. Non-conductive mesh can be made of plastic or othernon-conductive or low conductivity materials. In yet furtherembodiments, there may be a series of adjacent meshes with varyingelectrical conductivity. This structure will determine the localelectrical field which controls the local current distribution and hencethe plating uniformity.

FIG. 9 shows another structure suitable for inducing mixing in laminaror turbulent electrolyte flow. A small section of non-woven wire mesh830 is shown on part of the surface of the electrode 810. The directionof electrolyte flow is indicated by the arrow; the flow being generallyparallel to the surface of the electrode 810. The wire mesh 830 disruptsthe fluid flow, inducing desirable mixing in laminar or turbulent flowover the electrode surface. Note that a similar effect may also beachieved by having the mesh close to, but not necessarily on, thesurface of the electrode 810. A suitable wire diameter will be between10% and 50% of the channel thickness. Multiple wires may be stacked orspaced across the cell channel to further enhance performance in someembodiments.

FIG. 10 shows yet another structure suitable for inducing mixing inlaminar or turbulent electrolyte flow. Parallel wires/tubes 840 areshown on part of the surface of the electrode 810. The direction ofelectrolyte flow is indicated by the arrow; the flow being generallyparallel to the surface of the electrode 810 and perpendicular to thelong axes of the wires/tubes. The wires/tubes 840 disrupt the fluidflow, inducing desirable turbulent (non-laminar) flow over the electrodesurface. Note that a similar effect may also be achieved by having thewires/tubes close to, but not necessarily on, the surface of theelectrode 810. A suitable wire diameter will be between 10% and 90% ofthe channel thickness.

FIG. 11 shows part of an array of features for inducing mixing inlaminar or turbulent electrolyte flow. An array of cylinders 850 isshown on part of the surface of the electrode 810. The direction ofelectrolyte flow is indicated by the arrows; the flow being generallyparallel to the surface of the electrode 810. The array of cylinders 850disrupts the fluid flow, inducing desirable mixing in the flow over theelectrode surface. The cylinders as shown are formed of non-conductivematerial and may have sharp edges. A suitable cylinder height is between20% and 100% of the channel thickness. The spacing and diameter must besuch as to generate turbulence at the desired flow rate.

FIG. 12 shows part of another array of features for inducing mixing inlaminar or turbulent electrolyte flow. An array of cones (or taperedcylinders) 860 is shown on part of the surface of the electrode 810. Thedirection of electrolyte flow is indicated by the arrows; the flow beinggenerally parallel to the surface of the electrode 810. The array ofcones 860 disrupts the fluid flow, inducing desirable mixing in laminaror turbulent flow over the electrode surface. The tapered cylinders asshown are formed of non-conductive material and may have sharp points. Asuitable cylinder height is between 20% and 100% of the channelthickness. The spacing and diameter must be such as to generate mixingwhile not unduly increasing the flow resistance as the channel isreduced in thickness as the Zn deposit increases in thickness.

FIG. 13 shows part of yet another array of features for inducing mixingin the electrolyte flow. This is to illustrate that shapes other thancylinders are suitable. An array of pyramids 870 is shown on part of thesurface of the electrode 810. The direction of electrolyte flow isindicated by the arrows; the flow being generally parallel to thesurface of the electrode 810. The array of pyramids 870 disrupts thefluid flow, inducing desirable mixing of the flow over the electrodesurface. The tapered features as shown are formed of non-conductivematerial and may have sharp edges and may have other cross-sections, forexample triangular, other polygon or oval. A suitable feature height isbetween 20% and 100% of the channel thickness. The spacing and diametermust be such as to generate mixing at the desired flow rate. The tapermay be chosen to maintain a high rate of mixing while not undulyincreasing the flow resistance as the channel is reduced in thickness asthe Zn deposit increases in thickness.

FIGS. 8-13 provide a range of examples of features that may be used toinduce mixing in the electrolyte flow over the surface of the electrode.However, these examples are not intended to be a comprehensive listing,and further features suitable for inducing mixing in laminar and/orturbulent flow will be apparent to these skilled in the art afterreading this disclosure. For example, further features may include:combinations of the above described features; conductive andnon-conductive meshes; ribbons; foam structures; and other arrangementsof wires or tubes.

The arrays shown in FIGS. 11-13 are shown as regular arrays of features;however, these arrays may also have randomly positioned features, orpartially randomly positioned features.

The features of FIGS. 8-13 are shown on the surface of a flow batteryelectrode. However, such features may alternatively, or in addition, belocated in the electrolyte stream immediately prior to the electrolyteflowing across the electrode; the features may be attached, for exampleto the inner surface of the plumbing that delivers the electrolyte intothe half cell.

In commercial flow battery operation the power consumed by the pumpingsystem is an important factor in optimizing overall productivity of thebattery system. While high fluid pumping rates induce higher degrees ofmixing in the cell they also demand more pumping power that ultimatelydetracts from the power and energy delivered by the battery system.Higher pumping rates also causes higher wear and therefore more frequentpreventive maintenance. Pump power, mixing and turbulence can be tradedoff against each other in battery design. Laminar flow inside andoutside of the cell and through the pipes to and from the cell generallyreduces pumping power requirements. When operating with turbulence formixing the electrolyte in the flow channel of a cell, the turbulence canbe quenched for example by allowing for a lower velocity region in whichflow velocity is reduced and laminar flow resumed, for example usingstructures such as illustrated in FIG. 14. Ensuring flow outside of anyintentional turbulent region of the cell is laminar or substantiallylaminar reduces pump power consumption.

FIG. 14 shows a cross-section of a modified pipe at the exit from a halfcell, the pipe is designed to calm the turbulent flow and provide alaminar flow as the electrolyte moves through the rest of the plumbing.The turbulently flowing electrolyte flows from the half cell into afirst segment of pipe 1410. The electrolyte then enters a second segmentof pipe 1420 in which the cross-section of the pipe increases. As thepipe 1420 cross-section increases, the velocity of the electrolytedecreases and the turbulent flow is calmed, resulting in a laminar flow.The laminar flowing electrolyte then enters a third section of pipe 1430in which the pipe cross-section decreases, so as to funnel the calmedelectrolyte into the plumbing 1440 that continues the electrolytecircuit. The direction of electrolyte flow is indicated by the arrows.

The flow of electrolyte through the cell flow channel may be reversed toimprove mixing, uniformity of deposition and to avoid depletion ofelectrolyte at the deposition surface.

Standoffs can be used to support a mesh (or screen) mixing element inthe flow channel to avoid it contacting the membrane or electrode andgenerally to avoid bowing or buckling of the mesh due to high flow rate,turbulence or temperature variation.

Those of skill in the art will be aware of many definitions and measuresof turbulence. Turbulence in this context generally means variations inflow velocity (velocity being a vector, and variations including bothvariations in the speed and direction of flow) including to cause mixingof the electrolyte flow to avoid depletion of the deposited ion at orclose to the deposition surface during deposition (charging) or removalduring discharge.

Definition of substantially uniform deposition on a deposition surfaceof the cell during charging means deposition without or with reduceddendrite formation during the charging period. Those of skill in the artwill recognize that some variation of deposition thickness across thedeposition surface and across the cell (perhaps less than 20%) areinherent, particularly when in operation at high current density andwith high concentration of ions in the electrolyte.

A high rate of mixing generally means mixing in laminar or turbulentflow to avoid or minimize depletion of the plating ion particularly ator close to the deposition surface. A high rate of mixing for use withvarious embodiments can be achieved as follows: (1) with a channel andoff deposition surface mixing device; (2) with a channel and ondeposition surface mixing device; (3) with a channel with both on andoff deposition surface mixing devices; or (4) with a channel, mixingdevice and high electrolyte velocity.

First, a high rate of mixing may be achieved when the mixing element ordevice is located at a distance from the electrode deposition surface ofat least approximately twice the diffusion boundary layer thickness andhas a cross-sectional area of approximately 10% to 80% of the cellchannel cross-sectional area, desirably from approximately 25% to 60%,or a high rate of mixing can be achieved when the mixing device islocated at a distance from the electrode deposition surface of at leastapproximately 125 microns and has a cross-sectional area ofapproximately 10% to 80% of the cell channel cross-sectional area,preferably from approximately 25% to 60%; or a high rate of mixing canbe achieved when the mixing element or device is located at a distancefrom the electrode deposition surface of at least approximately twicethe diffusion boundary layer thickness and have a cross-sectional areaof approximately 10% to 80% of the cell channel cross-sectional area,preferably from approximately 25% to 60%, where the mixing device has arepeating feature (or approximately repeating feature) across the cellchannel width and along the cell channel length; or a high rate ofmixing can be achieved when the mixing element or device is located at adistance from the electrode deposition surface of at least approximatelytwice the diffusion boundary layer thickness and have a cross-sectionalarea of approximately 10% to 80% of the cell channel cross-sectionalarea, preferably from approximately 25% to 60%, where the mixing devicehas a repeating feature (or approximately repeating feature) across thecell channel width and along the cell channel length and the spacinginterval of the repeating feature along the cell channel length is atleast approximately 1.1 times the spacing interval of the repeatingfeature across the channel width.

Second, a high rate of mixing can be achieved when a mixing element ordevice or devices are attached to the electrode deposition surface andthe ratio of the mixing device leading edge repeat distance is at leastapproximately five times the mixing device height from the electrodedeposition surface, and the shape of the mixing device can be selectedfrom the group comprising a wire, mesh, screen, a semi-spherical, round,semi-round or rectangular shape or other shapes or combinations.

Third, a good rate of mixing can be achieved when a mixing element ordevice or devices are attached to the electrode deposition surface incombination with a second mixing device that is located at a distancefrom the electrode deposition surface of at least approximately twicethe diffusion boundary layer thickness and has a cross-sectional area ofapproximately 10% to 80% of the cell channel cross-sectional area,preferably from approximately 25% to 60%.

Fourth, a high rate of mixing can be achieved when the mixing element ordevice is located at a distance from the electrode deposition surface ofat least approximately twice the diffusion boundary layer thickness andhave a cross-sectional area of approximately 10% to 80% of the cellchannel cross-sectional area, preferably from 25% to 60%, and theelectrolyte fluid velocity is at least approximately 5 cm/s, preferablyat least approximately 25 cm/s and more preferably at leastapproximately 50 cm/s.

In some applications, flow batteries, such as some embodiments of thepresent invention, may be used for frequency regulation. Furthermore,some embodiments of the present invention may be used for other shortduration power needs such as UPS (uninterruptable power system) or shortresponse power backup. For short duration power needs some embodimentsof flow batteries of the present invention may be operable at highcharging and discharging current densities, such as greater than roughly200 mA/cm².

In other embodiments the flow battery may include one or more of thefollowing: a Reynolds Number of the flow channel is greater thanapproximately 1300; a Sherwood Number of the flow channel is greaterthan approximately 21; the uniform high current density is >100 mA/cm²;there is at least one mixing element in the flow channel; there is atleast one turbulence inducing element in the flow channel; the masstransfer coefficient is greater than approximately 7.7×10⁻⁴ m/s; and thecharging cycle is at least 5 minutes in duration or is at least one hourin duration.

In some embodiments, the high performance flow battery may comprise astack of cells with a sustainable operating current density in a regionof a cell in the cell stack during a charging cycle of >100 mA/cm². Insome embodiments, the high performance zinc-based flow battery maycomprise depositing zinc on a deposition surface in a cell of thebattery at a rate greater than 0.1 mm per hour, and in other embodimentsat a rate greater than 0.2 mm per hour. Furthermore, in some embodimentsa method of charging a high performance ZnFe flow battery may comprisegrowing or depositing zinc on a deposition surface of a cell in thebattery at a rate greater than 0.1 mm per hour, and in other embodimentsat a rate greater than 0.2 mm per hour or greater than 0.4 mm per hour.

In another embodiment, a high performance redox flow battery comprisesat least one cell comprising a low resistance positive electrode in atleast one positive half cell and a low resistance negative electrodes inat least one negative half cell where a resistance of the low resistancepositive electrode and a resistance of the low resistance negativeelectrode is small enough for uniform high current density across aregion of a deposition surface of the at least one cell, an electrolyteflow through a flow channel of at least one half cell with a high rateof mixing in a deposition region proximate the deposition surface wherethe electrolyte has sufficiently high concentration of an active ionspecies for deposition rates on the deposition surface that sustain theuniform high current density through the deposition surface during acharging cycle, a mass transfer coefficient of the flow proximate thedeposition surface at least enough to maintain sufficient electrolyteconcentration proximate the deposition surface for substantially uniformdeposition in the region of the deposition surface.

In another embodiment, a method of charging a high performance flowbattery comprising delivering a sufficient supply of electrical energyto the flow battery at a voltage higher than a voltage of the flowbattery, providing a uniform high current density across a lowresistance positive electrode and a low resistance negative electrode,the high current density passing through a region of a depositionsurface of at least one half cell of the flow battery, generating anelectrolyte flow through a flow channel of the at least one half cellwith a high rate of mixing in a deposition region proximate thedeposition surface where the electrolyte has sufficiently highconcentration of an active ion species for deposition rates on thedeposition surface that sustain the uniform high current density throughthe deposition surface during the charging, maintaining a mass transfercoefficient of the flow proximate the deposition surface sufficientlylarge to maintain a sufficient electrolyte concentration proximate thedeposition surface for substantially uniform deposition in the region ofthe deposition surface. The flow may be laminar or turbulent. The highrate of mixing is in the range sufficient to maintain the mass transfercoefficient greater than 7.7×10⁻⁴ cm/s.

Furthermore, the present invention includes a method of optimizing ahigh performance redox flow battery comprising engineering a flow rateand a flow channel of the flow battery optimized to ensure one or moreof the following parameters are satisfied: uniform mass transfer rateacross the deposition surface, at optimal fluid velocity; local currentdensity of less than approximately ⅔×i_(L), but sufficiently high toprevent mossy deposition (in other words a local current densitysuitable for providing, a dense, adherent, non-dendritic morphology);and concentration depletion along the flow channel (on the Zn side) toapproximately <10% of an inlet concentration.

Experiments confirm (1) a zinc solubility of 0.73M in 4N NaOH and (2) alimiting current density (at a rotating disk electrode) for thissolution of 121 mA/cm² at a rotation speed of 120 rpm at 40° C.

The super-saturated zinc electrolyte—0.73M Zn++ in 4N NaOH—was preparedas follows. Step 1: prepare a stock solution (1 M Zn⁺⁺ in 5.5N NaOH) bycombining 8.139 g of ZnO (m.w. 81.39 g/mol) with 30 gm of NaOH pellets(m.w. 40 g/mol) and making up to 100 ml with D.I. water under constantstirring. The resulting solution is a 1M Zn⁺⁺5.5N NaOH. Step 2: dilutionof (1 M Zn⁺⁺/5.5N NaOH) to a 4N NaOH solution by taking 100 ml of (1MZn⁺⁺ in 5.5N NaOH) stock solution and making it up to 137.5 ml with D.I.water—the resulting solution is 0.73M Zn⁺⁺ in 4N NaOH. (Note that thereported solubility limit of Zn⁺⁺ in 4N NaOH is 0.37M.) Note thatelectrolytes with NaOH concentrations in the range of 2-4N are found toprovide satisfactory zincate ion concentration in combination withtolerable ferrous ion concentration and tolerable corrosive solutionproperties, whereas NaOH concentrations above 4N result in rapidlyreduced ferrous ion concentrations along with an electrolyte which ismore corrosive.

Experiments confirm that 0.73M Zn⁺⁺ in 4N NaOH and 0.4 Zn⁺⁺ in 2.2N NaOHare stable for at least four weeks.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

1. A method of charging a flow battery, comprising: providing a uniformhigh current density across a low resistance positive electrode and alow resistance negative electrode, the high current density passingthrough a deposition region of a deposition surface of a negativehalf-cell of the flow battery; generating a super-saturated electrolyteflow through a flow channel of the negative half cell with a high rateof mixing in a deposition region proximate the deposition surface,wherein the super-saturated electrolyte has a zinc ion concentrationgreater than 0.4N; and maintaining a mass transfer coefficient of theflow proximate the deposition surface sufficiently large to maintain asufficient electrolyte concentration proximate the deposition surfacefor substantially uniform deposition in the region of the depositionsurface.
 2. The method of claim 1, wherein the super-saturatedelectrolyte has a sufficient concentration of zinc ions for depositionrates on the deposition surface that sustains the uniform high currentdensity through the deposition surface during the charging.
 3. Themethod of claim 1, wherein the super-saturated electrolyte has a zincsolubility of greater than about 0.7M in a 4N NaOH containing solution.4. The method of claim 1, wherein the super-saturated electrolyte has azinc solubility of about 0.73M in a 4N NaOH containing solution.
 5. Themethod of claim 1, wherein the super-saturated electrolyte is preparedby combining zinc oxide (ZnO) with NaOH pellets.
 6. The method of claim1, wherein the uniform high current density is greater than 70 mA/cm2.7. The method of claim 1, wherein a mass transfer coefficient of thesuper-saturated electrolyte has a value in the approximate range of5.3×10−4 m/s to 12.4×10−3 m/s.
 8. The method of claim 1, wherein theflow battery is a flow battery selected from the group consisting of: aZnFe flow battery, a ZnHBr flow battery, a ZnBr flow battery, a CeZnflow battery; and a ZnCl flow battery.
 9. The method of claim 1, whereinthe flow channel is configured to provide a high rate of mixing of thesuper-saturated electrolyte in the negative plating zone proximate thesurface of the negative electrode.